Circuit and method for receiving and mixing radio frequencies in a direct conversion receiver

ABSTRACT

A frequency mixing circuit and a frequency mixing method. The frequency mixing circuit includes first and second differential amplifiers, a subtracter and a mixer. The first differential amplifier amplifies a first pair of input signals having a first frequency to generate a first differential signal. The second differential amplifier amplifies a second pair of input signals having the first frequency orthogonal to the first pair input signals to generate a second differential signal. The subtracter subtracts the second differential signal from the first differential signal. The mixer mixes the subtracted signal with a first and second pairs of drive signals having a second frequency orthogonal to each other, in a sub-harmonic double balanced mixing mode, so that the mixer generates a pair of output signals orthogonal to each other without secondary harmonics.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to direct conversion receivers andcircuits and methods for receiving and mixing radio frequencies, andmore particularly to a circuit and a method that can diminish asecondary-order inter modulation distortion at a direct conversionreceiver.

[0003] 2. Description of the Related Art

[0004] Generally, the direct conversion receiver or a homodyne receiverprovides advantages compared to a superheterodyne receiver.

[0005]FIG. 1 is a radio frequency receiving circuit diagram of aconventional direct conversion receiver (DCR).

[0006] Referring to FIG. 1, the direct conversion receiver transforms aninput signal into an inphase signal and a quadrature signal having abaseband frequency without transforming the input signal into a signalhaving an intermediate frequency (IF).

[0007] A radio frequency (RF) signal received by an antenna 10 isinputted to a low noise amplifier 12, and then an output signal of thelow noise amplifier 12 is inputted to each of a first mixer 14 and asecond mixer 16.

[0008] At the first mixer 14, the amplified radio frequency signal ismixed with a local oscillator signal 20 such as a cosine wave signal ofa local oscillator 20. The local oscillator signal 20 has a samefrequency as a carrier frequency. At the second mixer 16, the radiofrequency signal is mixed with a sine wave. The sine wave has phasedifference of 90° with respect to the local oscillator signal 20, andgenerated by a π/2 phase shifter 18.

[0009] The first and second mixers 14 and 16 generate the inphase signaland the quadrature signal, respectively, which have a mean frequencysuch as the baseband frequency and a harmonic frequency such as a twicecarrier frequency (2 fc). Harmonics of the signals generated by thefirst and second mixers 14 and 16 are removed by two low pass filters 22and 24, respectively. The inphase signal and the quadrature signalhaving baseband frequency are amplified and outputted by two amplifiers26 and 28, respectively.

[0010] The DCR has a simple circuit configuration compared with thesuperheterodyne receiver, and is easier to implement as an integratedcircuit. The a minimized DCR circuit can be manufactured at a low cost.

[0011] However, the DCR has some problems. One of the problems is asecondary intermodulation distortion generated by the mixer. Thesecondary intermodulation distortion is caused by the mixer having anonlinear active device. A harmonic frequency component of the outputsignal is generated by a radio frequency signal process using thenonlinear active device, and may be a sum or difference of the harmonicsof two different input signals. A DC offset is generated in addition tounwanted secondary harmonics by a non-linearity of the mixer.

[0012] When two input signals respectively having two frequencycomponents f1 and f2 are inputted into a nonlinear circuit, frequencycomponents such as 2f1, 2f2, f1+f2, 3f1, 3f2, 2f1−f2, 2f2−f1, 2f1+f2 or2f2+f1 are generated due to the non-linearity of the nonlinear circuitas well as f1, f2. In general, a filter removes the frequency componentscaused by a non-linearity.

[0013] When the input signal frequencies f1 and f2 are slightlydifferent from each other and an application defines the basebandfrequency as the mean frequency, the frequency component of f1-f2 thatis close to the baseband frequency is not removed by the filter. Thefrequency component due to the non-linearity is presented in the form ofinterference among channels having a small frequency difference, or insignal distortions by mutual interference of the signals in a signalband.

[0014] The frequency component of f1-f2 is referred to as the secondaryintermodulation distortion (IMD2). The linearity of circuit isrepresented by a relation between the IMD2 quantity and a quantity of anamplified input signal frequency. A value representing the linearity ofcircuit is referred to as a second order intercept point (IP2).

[0015] Additionally, because the DCR shifts the frequency of the desiredsignal to the baseband, the IMD2 generated by the mixer can deterioratethe function of the DCR.

[0016] To solve above mentioned problem, some attempts have beensuggested.

[0017] One of the attempts is to control mismatches of load resistancesto equalize phases of the outputted secondary harmonics, and to equalizeamplitudes of the outputted secondary harmonics, so that the secondaryharmonics is removed by differential inputs. The effectiveness of themethod of the matching load resistances depends on how finely the loadresistances are controlled. However, the precise control of the loadresistances is limited by a fabrication process of the integratedsemiconductor circuit.

[0018] Another method is disclosed in Korean Patent Laid-OpenPublication Nos. 2001-34820 (that corresponds to U.S. patent applicationSer. No. 09/064,930), and 2002-68128.

[0019] In Korean Patent Laid-Open Publication No. 2001-34820, the IMD2is transformed out of a pass band of a low pass filter and removed, by aswitching operation of an inverter for an outputted signal polarity of amixer,. In addition, a switching frequency of the inverter is high ascompared with bandwidth of an input signal.

[0020] According to the disclosure in Korean Patent Laid-OpenPublication No. 2002-68128, the IMD2 is minimized by circuitconfiguration biased in region in which a first differential function oftransconductance of a complementary active device has a maximum andminimum values.

SUMMARY OF THE INVENTION

[0021] The present invention provides a frequency mixing circuit and afrequency mixing method for removing a secondary intermodulationdistortion (IMD2) that improves linearity.

[0022] It is another aspect of the present invention to provide a radiofrequency receiving circuit and a radio frequency receiving method usingthe frequency mixing circuit and the frequency mixing method.

[0023] In one aspect of the present invention, the first embodiment ofthe frequency mixing circuit includes a first differential amplifier, asecond differential amplifier, a subtracter and a mixer. The firstdifferential amplifier amplifies a first pair of input signals RF1 andRF2 with a first frequency f1 to generate a first differential outputsignal. The second differential amplifier amplifies a second pair ofinput signals RF3 and RF4 orthogonal to the first pair input signals RF1and RF2 to generate a second differential output signal. The subtractersubtracts the second differential output signal from the firstdifferential output signal, so that the subtracter generates asubtracted signal. The mixer mixes the subtracted signal, a first pairof drive signals L01 and L02 and a second pair of drive signals L03 andL04 orthogonal to each other, in a sub-harmonic double balanced mixingmode, so that the mixer generates a pair of output signals orthogonal toeach other without secondary harmonics.

[0024] In the second embodiment the frequency mixing circuit has thesame circuit configuration as the first embodiment of the frequencymixing circuit, except that the mixer includes a Gilbert cell circuit inplace of a sub-harmonic double balanced mixing circuit of the firstembodiment.

[0025] In the third embodiment, the frequency mixing circuit includesone differential amplifier, a harmonic rejection circuit and a mixer.The differential amplifier amplifies a first pair of input signals RF1and RF2 having a first frequency f1, so that the differential amplifiergenerates a first current signal at a first node and a second currentsignal at a second node. The harmonic rejection circuit reacts to asecond pair of input signals RF3 and RF4 orthogonal to each other,having a substantially same frequency as the first frequency f1, so thatthe harmonic rejection circuit generates a third current signal at thefirst node and a fourth current signal at the second node. The mixermixes the current signals at the first and second nodes, with a firstpair of drive signals L01 and L02 and a second pair of drive signals L03and L04 (orthogonal to the first pair of drive signals L01 and L02)having a second frequency f2, in a sub-harmonic double balanced mixingmode, so that the mixer generates a pair of output signals orthogonal toeach other.

[0026] In one embodiment of the frequency mixing method, the methodincludes generation of a first differential signal, a seconddifferential signal, a subtracted signal, and a pair of output signals.The first differential signal is produced by amplifying a first pair ofinput signals with a first frequency. The second differential signal isproduced by amplifying a second pair of input signals having asubstantially same frequency as the first pair input signals, and isorthogonal to the first pair input signals. The subtracted signal isproduced by subtracting the second differential signal from the firstdifferential signal. The pair of output signals is produced by mixingthe subtracted signal, with a first pair of drive signals and a secondpair of drive signals having a second frequency. The mixing process usesa sub-harmonic double balanced mode so that the pair of output signal isorthogonal to each other and secondary harmonics are removed.

[0027] In another embodiment of the frequency mixing method, the methodincludes generation of a first differential signal, a seconddifferential signal, a subtracted signal, and a pair of output signalsby using a double balanced mixing mode. The first differential signal isproduced by amplifying a first pair of input signals having a firstfrequency. The second differential signal is produced by amplifying asecond pair of input signals having a substantially same frequency asthe first frequency, and is orthogonal to the first pair input signals.The subtracted signal is produced by subtracting the second differentialsignal from the first differential signal. The pair of output signals isproduced by mixing the subtracted signal with a pair of drive signalshaving a second frequency. The mixing method uses a sub-harmonic doublebalanced mode so that the pair of output signal are orthogonal to eachother, and a secondary harmonic is removed.

[0028] In still another embodiment of the frequency mixing method, themethod includes generation of a first current signal and a secondcurrent signal, a first subtracted signal and a second subtractedsignal, and a pair of output signals. The first and second currentsignals are produced by amplifying a first pair of input signals RF1 andRF2 having a first frequency f1. The first and second subtracted signalsare produced by respectively subtracting a third current signal and afourth current signal from the first and second current signals. Thesubtraction is an operation responding to a second pair of input signalsRF3 and RF4 that have a substantially same frequency as the firstfrequency and are orthogonal to the first pair of input signals. Thepair of output signals is produced by mixing the first subtractedsignal, the second subtracted signal, a first and a second pair of drivesignals orthogonal to each other with a second frequency f2. The mixinguses a sub-harmonic double balanced mixing mode so that the pair ofoutput signals is orthogonal to each other.

[0029] In another aspect of the present invention, the first embodimentof the radio frequency receiving circuit includes a first poly-phasefilter, a second poly-phase filter, a first mixer and a second mixer.The first and second mixers have a sub-harmonic double balanced activemixer adapted to cancel harmonics. The first poly-phase filtertransforms a radio frequency signal having a first frequency into afirst and second pairs of input signals orthogonal to each other. Thesecond poly-phase filter transforms a local oscillator signal having asecond frequency into first and second signal groups that each includesa pair of drive signals having about 45°-phase difference from eachother. The first mixer is coupled to the first and second poly-phasefilters. Additionally, the first mixer mixes the two pairs of inputsignals and a pair of drive signals in the first group signal togenerate a first output signal having a third frequency. The secondmixer coupled to the first and second poly-phase filters mixes the twopairs of input signals and a pair of drive signals in the second groupsignal to generate a second output signal having a substantially samefrequency as the third frequency.

[0030] In another aspect of the present invention, the second embodimentof the radio frequency receiving circuit includes a first poly-phasefilter, a second poly-phase filter, a first mixer and a second mixer.The first or second mixer has a double balanced active mixing circuitthat is widely known as a Gilbert cell circuit. The first poly-phasefilter transforms a radio frequency having a first frequency into twopairs of input signals orthogonal to each other. The second poly-phasefilter transforms a local oscillator signal having a second frequencyinto two pairs of drive signals orthogonal to each other. The firstmixer coupled to the first and the second poly-phase filters mixes thetwo pairs of input signals and one pair of drive signals to generate afirst output signal having a third frequency. The second mixer coupledto the first and the second poly-phase filters mixes the two pairs ofinput signals and the other pair of drive signals to generate a secondoutput signal having a substantially same frequency as the thirdfrequency.

[0031] In another aspect of the present invention, one embodiment of theradio frequency receiving method includes generation of two pairs ofinput signals, the first and second signal groups, a first output signaland a second output signal. The two pairs of input signals are producedby transforming a radio frequency signal, and are orthogonal to eachother. The first and second signal groups are produced by transforming alocal oscillator signal. Additionally, the first and second signalgroups have about 45°-phase difference from each other and each signalgroup has two pairs of drive signals orthogonal to each other. The firstoutput signal is produced by mixing the two pairs of input signals andthe two pairs of the first group's signals. The first output signal hasa third frequency. The second output signal is produced by mixing thetwo pairs of input signals and the two pairs of the second groupsignals. The second output signal has a substantially same frequency asthe third frequency.

[0032] In another aspect of the present invention, another embodiment ofthe radio frequency receiving method includes generation of two pairs ofinput signals, two pairs of drive signals, a first output signal, and asecond output signal. The two pairs of input signals are generated bytransforming a radio frequency signal having a first frequency. The twopairs of input signals are orthogonal to each other. The two pairs ofdrive signals are generated by transforming a local oscillator signalhaving a second frequency, so that the two pairs of drive signals areorthogonal to each other. The first output signal is produced by mixingthe two pairs of input signals and one pair of drive signals. The firstoutput signal has a third frequency. The second output signal isproduced by mixing the two pairs of input signals and the other pair ofdrive signals. The second output signal has a substantially samefrequency as the third frequency.

[0033] The present invention renders the secondary intermodulationdistortion IMD2 reduced by changing input structure of the mixingcircuit that receives the radio frequency signal, so that removing thesecondary harmonics improves the linearity of the mixing circuit and thequality of receiving circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The above and other features of the present invention will becomemore apparent by describing in detail the preferred embodiments thereofwith reference to the accompanying drawings, in which:

[0035]FIG. 1 is a circuit diagram of a conventional radio frequencyreceiving direct conversion receiver;

[0036]FIG. 2 is a circuit diagram of a harmonic rejection mixing circuitaccording to a first embodiment of the present invention;

[0037]FIG. 3 is a circuit diagram of a harmonic rejection mixing circuitaccording to a second embodiment of the present invention;

[0038]FIG. 4 is a circuit diagram of a harmonic rejection mixing circuitaccording to a third embodiment of the present invention;

[0039]FIG. 5 is a phase diagram of drive signals shown in FIG. 6; and

[0040]FIG. 6 is a block diagram of a radio frequency receiving circuitaccording to an exemplary embodiment of the present invention;

[0041]FIG. 7 is a block diagram of a radio frequency receiving circuitaccording to another exemplary embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] Hereinafter, the preferred embodiments of the present inventionwill be described in detail with reference to the accompanying drawings.

[0043] Exemplary Embodiments as a Frequency Mixing Circuit

[0044] Embodiment 1

[0045]FIG. 2 is a circuit diagram of a harmonic rejection mixing circuitaccording to a first embodiment of the present invention.

[0046] Referring to FIG. 2, a frequency mixing circuit 100 comprises afirst differential amplifier 110, a second differential amplifier 120, asubtracter 130 and a mixer 140.

[0047] The first differential amplifier 110 has a pair of an emittercoupled transistors Q1 and Q2 that are emitter coupled at first commonnode CN1. The first transistor Q1 has a base receiving a first inputsignal RF1 and the second transistor Q2 has a base receiving a secondinput signal RF2. The first input signal RF1 and the second input signalRF2 are 180° out of phase with respect to each other, and become a firstpair input signals. The first differential amplifier 110 generates afirst amplified signal I_(RFQ0) by amplifying the first pair inputsignals RF1 and RF2. A first bias current source BCS1 is connectedbetween the first common node CN1 (of the emitter coupled transistors Q1and Q2) and a ground GND. The first bias current source BCS1 supplies abias current I_(t) to the first common node CN1. A first regenerationresistor R1 is connected between the first common node CN1 and theemitter of the transistor Q1. A second regeneration resistor R2 isconnected between the first common node CN1 and the emitter of thetransistor Q2. The first and second regeneration resistors R1 and R2 area matching pair.

[0048] The second differential amplifier 120 has a pair of an emittercoupled transistors Q3 and Q4 that are emitter-coupled at a secondcommon node CN2. One of the emitter coupled transistors (Q3) has a basereceiving a third input signal RF3, and the other of the emitter coupledtransistors (Q4) has a base receiving a fourth input signal RF4. Thethird input signal RF3 and the fourth input signal RF4 are 180° out ofphase with respect to each other, and become a second pair of inputsignals. The second differential amplifier 120 generates a secondamplified signal I_(RF10) by amplifying the second pair input signalsRF3 and RF4. Additionally, the second pair input signals are 90° out ofphase with respect to the first pair input signals. A second biascurrent source BCS2 is connected between a second common node CN2 of theemitter coupled transistors Q3 and Q4 and the ground GND. The secondbias current source supplies a bias current I_(t) to the second commonnode CN2. A third regeneration resistor R3 is connected between thesecond common node CN2 and the emitter of the transistor Q3. A fourthregeneration resistor R4 is connected between the second common node CN2and the emitter of the transistor Q4. The third and fourth regenerationresistors become a matching pair.

[0049] The subtracter 130 has a first transformer T1, a secondtransformer T2 and a third current source BCS3. The subtracter 130generates a subtraction signal I_(RF0) by subtracting the secondamplified signal I_(RF10) from the first amplified signal I_(RFQ0).

[0050] The first transformer T1 has a first winding W1 and a secondwinding W2 that are magnetically coupled to each other and have the samepolarity with respect to each other. One terminal of the first windingW1 is connected to the collector of transistor Q1, and the otherterminal of the first winding W1 is connected to the collector oftransistor Q2. A center tap is connected to a voltage source VCC. Thefirst amplified signal I_(RFQ0) at the first winding W1 is inductivelycoupled to the second winding W2.

[0051] The second transformer T2 has a third winding W3 and a fourthwinding W4 that are magnetically coupled to each other and have anopposite polarity with respect to each other. A polarity of the thirdwinding W3 is opposite to the polarity of the first winding W1 of thefirst transformer T1. The polarity of the fourth winding W4 is same withthe polarity of the second winding W2. One terminal of the third windingW3 is connected to the collector of transistor Q3, and the otherterminal of the third winding W3 is connected to the collector oftransistor Q4. The center tap of the third winding W3 of the secondtransformer T2 is connected to the voltage source VCC. The secondamplified signal I_(RF10) at the third winding W3 is inductively coupledto the second winding W4.

[0052] One terminal of the second winding W2 is connected to the mixer140, and the other terminal of the second winding W2 is connected to athird common node CN3. One terminal of the fourth winding W4 isconnected to the third common node CN3 and the other terminal of thefourth winding W4 is connected to the mixer 140. A third bias currentsource BCS3 is connected between the third common node and the groundGND. The third bias current source BCS2 applies DC current to the mixer140.

[0053] Thus, a subtraction of the first amplified signal and the secondamplified signal is performed by a coupled configuration of the firstand the second transformers T1 and T2. A circuit configuration for thesubtraction using the transformers can be operated at a low voltage andcan minimize leakage current characteristics.

[0054] The mixer 140 is a sub-harmonic double balanced mixing circuithaving four frequency multipliers FD1, FD2, FD3 and FD4. Each of thefrequency multipliers comprises a pair of transistors that havecollectors commonly connected to each other and emitters commonlyconnected to each other. In the sub-harmonic double balanced mixingcircuit 140, a drive signal frequency f2 of four drive signalsL01,L02,L03 and L04 is half of the input signal frequency f1 of theinput signals RF1, RF2, RF3 and RF4. A first pair of drive signals L01and L02 is 180° out of phase with respect to each other. A second pairof drive signals L03 and L04 is 180° out of phase with respect to eachother. The first and second pair of drive signals are orthogonal to eachother. The two pairs of drive signals are mixed at the mixer 140.

[0055] The mixer 140 has harmonic having a frequency of f1-2f2.

[0056] The first frequency multiplier FD1 has the collectors commonlyconnected to a first output node ON1 and the emitters commonly connectedto one terminal of the second winding W2. A first base of the firstfrequency multiplier FD1 receives the first drive signal L01 having 0°phase. A second base of the first frequency multiplier FD1 receives thesecond drive signal L02 having about 180°-phase difference compared tothe first drive signal L01.

[0057] The second frequency multiplier FD2 has collectors commonlyconnected to a second output node ON2 and the emitters commonlyconnected to one terminal of the second winding W2. A first base of thesecond frequency multiplier FD2 receives the third drive signal L03having about 90°-phase difference compared to the first drive signalL01. A second base of the second frequency multiplier FD1 receives thefourth drive signal L04 having about 270°-phase difference compared tothe first drive signal L01.

[0058] The third frequency multiplier FD3 has collectors commonlyconnected to a first output node ON1 and the emitters commonly connectedto one terminal of the fourth winding W4. A first base of the thirdfrequency multiplier FD3 receives the fourth drive signal L04 havingabout 270°-phase difference compared to the first drive signal L01. Asecond base of the third frequency multiplier FD3 receives the thirddrive signal L03 having about 90°-phase difference compared to the firstdrive signal L01.

[0059] The fourth frequency multiplier FD4 has collectors commonlyconnected to a second output node ON2 and the emitters commonlyconnected to one terminal of the fourth winding W4. A first base of thefourth frequency multiplier FD4 receives the second drive signal L02having about 180°-phase difference compared to the first drive signalL01. A second base of the fourth frequency multiplier FD4 receives thefirst drive signal L01 having 0° phase.

[0060] A first load resistor R5 is connected between the voltage sourceVCC and the first output node ON1, and a second load resistor R6 isconnected between the voltage source VCC and the second output node ON2.A capacitor C is coupled between the first output node ON1 and thesecond output node ON2.

[0061] Thus, in this embodiment, a secondary intermodulation distortion(IMD2) is minimized by the subtracter implemented by the RF transformer130. A first output signal IF1 is output from the first output node ON1;and a second output signal IF2 is output from the second output nodeON2. The first and second output signals IF1 and IF2 have about180°-phase difference from each other.

[0062] Embodiment 2

[0063]FIG. 3 is a circuit diagram of a harmonic rejection mixing circuitaccording to a second embodiment of the present invention.

[0064] A frequency mixing circuit shown in FIG. 3 has the sameconfiguration as the first embodiment of the frequency mixing circuit asshown in FIG. 2, except for the mixer 240. Therefore, in FIG. 3, thesame reference numerals denote the same elements in FIG. 2, and thus thedetailed description of the same elements will be omitted.

[0065] Referring to FIG. 3, the mixer 240 has a double balanced mixingcircuit including a Gilbert cell circuit. Thus, the frequency of drivesignals L01 and L02 is the same as the frequency of input signals RF1,RF2, RF3 and RF4.

[0066] A first pair of emitter coupled transistors Q5 and Q6 hasemitters commonly connected to each other and connected to one terminalof the second winding W2. The collector of one of the first pair ofemitter coupled transistors Q5 and Q6 is connected to the first outputnode ON1, and the collector of the other one of the first pair ofemitter coupled transistors Q5 and Q6 is connected to the second outputnode ON2. Furthermore, the first pair of emitter coupled transistors Q5and Q6 has a first base and a second base. The first base receives thefirst drive signal L01 having 0° phase and the second base receives thesecond drive signal L02 having about 180°-phase difference with respectto the first drive signal L01.

[0067] The second pair emitter coupled transistors Q7 and Q8 hasemitters commonly connected to each other and connected to one terminalof the fourth winding W4. A third collector, of one of a second pair ofemitter coupled transistors Q7 and Q8 is connected to the first outputnode ON1, and a fourth collector of the other one of the second pair ofemitter coupled transistors Q7 and Q8 is connected to the second outputnode ON2. Furthermore, the second pair emitter coupled transistors Q5and Q6 has a third base and a fourth base. The third base receives thesecond drive signal L02 having 180° phase difference with respect to thefirst drive signal L01, and the fourth base receives the first drivesignal L01 having 0° phase.

[0068] Embodiment 3

[0069]FIG. 4 is a circuit diagram of a harmonic rejection mixing circuitaccording to a third embodiment of the present invention.

[0070] Referring to FIG. 4, a frequency mixing circuit 400 has adifferential amplifier 410. The differential amplifier 410 amplifies afirst pair of input signals RF1 and RF2 in order to output a firstcurrent signal I_(RF1) and a second current signal I_(RF2). =A currentto flow at a first node N1 into transistor Q13 is the first currentsignal I_(RF1) and a current to flow at a second node N2 into transistorQ14 is the second current signal I_(RF2). The differential amplifier 410has a pair of emitter coupled transistors Q13 and Q14 and a bias currentsource BCS7. Transistor Q13 has a collector connected to the first nodeN1, a base receiving the first input signal RF1 having 0° phase, and anemitter connected a common node CN4 via a regeneration resistor R7.Transistor Q14 has a collector connected to the second node N2, a basereceiving the second input signal RF2 having about 180°-phase differencewith respect to the first input signal RF1, and an emitter connected acommon node CN4 via a regeneration resistor R8.

[0071] The bias current source BCS7 supplies a DC bias current 2I_(t) tothe common node CN4, and is connected between the common node CN4 and aground GND.

[0072] A harmonic rejection circuit 420 comprises a pair of transistorsQ15 and Q16 and bias current sources BCS8-BCS11.

[0073] The transistor Q15 has an emitter connected to the first node N1,a base receiving a third input signal RF3 having about 90°-phasedifference with respect to the first input signal RF1, and a collectorconnected to a voltage source VCC via the bias current source BCS8.Additionally, the bias current source BCS9 is connected between thefirst node N1 and the ground GND.

[0074] The transistor Q16 has an emitter connected to the second nodeN2, a base receiving a fourth input signal RF4 having about 270°-phasedifference with respect to the first input signal RF1, and a collectorconnected to the voltage source VCC via the bias current source BCS10.Additionally, the bias current source BCS11 is connected between thesecond node N2 and the ground GND.

[0075] DC current values of the bias current sources BCS8-BCS11 are thesame.

[0076] The transistor Q15 is turned on when the third input signal RF3has positive value, and the transistor Q13 is turned on when the firstinput signal RF1 has positive value, so that the first current signalI_(RF1) and a third current signal IRF3 have the opposite currentdirection from each other. The third input signal RF3 has about90°-phase delay from the first input signal RF1. Thus, while thetransistor Q13 is turned off, the transistor Q15 is turned on, so that acomplementary current operation at the first node N1 occurs.Consequently, a current I_(RE01) of the first node N1 is given by

I _(RE01) =I _(t)+(I _(RF1) −I _(RF3)).

[0077] In the same manner, a current I_(RE02) of the second node N2 isgiven by

I_(RE02) =I _(t)+(I _(RF2) −I _(RF4)).

[0078] In this way, a mixer 140 receives a signal of which a secondaryharmonics is removed by a subtraction for the input signals.

[0079] Exemplary Embodiments as a Frequency Receiving Circuit

[0080] Embodiment 5

[0081]FIG. 6 is a block diagram of a radio frequency receiving circuitaccording to an exemplary embodiment of the present invention.

[0082] Referring to FIG. 6, a radio frequency signal RF of the radiofrequency received to circuit 500 is transmitted into a first poly-phasefilter 530 through a low noise amplifier 510 and a transformer 520. Thefirst poly-phase filter 530 receives the radio frequency signal RF, andoutputs a first pair of input signals RF1 and RF2 and a second pair ofinput signals RF3 and RF4. The first and second pairs of input signalsare orthogonal to each other, so that the input signals RF1, RF2, RF3and RF4 have phase difference of about 0°, 90°, 180° and 270° withrespect to the input signal RF1, respectively.

[0083] Meanwhile, a local oscillator signal LO received at a secondpoly-phase filter 540 is transformed into a first signal group GS1 and asecond signal group GS2 by separating the local oscillator signal LO.The first signal group GS1 has signals having phases of 0°, 90°, 180°and 270°. The second group GS2 has signals having phases of 45°, 135°,225° and 315°. The signals of the first signal group GS1 have 45°-phasedifference from the signals of the second signal group GS2,respectively.

[0084] The circuit configuration of the poly-phase filters 530 and 540can be the same as or different from the poly-phase filter disclosed inU.S. Patent Laid-Open Publication No. 2001-38323.

[0085] The first signal group GS1 comprises a first pair of drivesignals L01 and L02 and a second pair of drive signals L03 and L04. Thefirst pair of drive signals L01 and L02 are orthogonal to the secondpair of drive signals L03 and L04. The second signal group GS2 has athird pair of drive signals L05 and L06 and a fourth pair of drivesignals L07 and L08. The third pair of drive signals L05 and L06 areorthogonal to the fourth pair of drive signals L07 and L08. phasedifference among the drive signals is shown by a phase diagram depictedin FIG. 5.

[0086] A first mixer 550 receives input signals RF1-RF4 and generates afirst intermediate frequency signal IF1 by mixing input signals RF1-RF4with frequency of the drive signals L01-L04 of the first signal groupGS1.

[0087] A second mixer 560 receiving the input signals RF1-RF4 andgenerates a second intermediate frequency signal IF2 by mixing inputsignals RF1-RF4 with frequency of the drive signals L05-L08 of thesecond group signal GS2.

[0088] The first and second mixers 550 and 560 each comprise thesub-harmonic double balanced mixing circuit disclosed in the first,third and other embodiments of the frequency mixing circuit.

[0089] The first intermediate frequency signal IF1 generated by thefirst mixer 550 is amplified and low-pass filtered by a first amplifier570, and whose DC offset is removed, so that a signal I inphase with thefirst intermediate frequency signal IF1 is generated.

[0090] The second intermediate frequency signal IF2 generated by thesecond mixer 560 is amplified and low-pass filtered by a secondamplifier 580, and whose DC offset is removed, so that a signal Qorthogonal to the signal I is generated. Furthermore, the signal I andthe signal Q have a baseband frequency.

[0091] Embodiment 6

[0092]FIG. 7 is a block diagram of a radio frequency receiving circuitaccording to another exemplary embodiment of the present invention.

[0093] The radio frequency receiving circuit 600 shown in FIG. 7 has thesame configuration as the radio frequency receiving circuit as shown inFIG. 6, except that a second poly-phase filter, a first mixer and secondmixer have a different configuration from the embodiment of the radiofrequency receiving circuit shown in FIG. 6. Therefore, in FIG. 7, thesame reference numerals denote the same elements in FIG. 6, and thus thedetailed description of the same elements will be omitted.

[0094] Referring to FIG. 7, a second poly-phase filter 640 receiving alocal oscillator signal LO generates a first pair of drive signals L01and L02 and a second pair of drive signals L03 and L04 by separating thelocal oscillator signal LO. The first and second pairs of drive signalsare orthogonal to each other. Furthermore, a frequency f2 of the localoscillator signal LO is the same as a frequency f1 of the radiofrequency signal RF.

[0095] A first mixer 650 receiving input signals RF1-RF4 generates afirst intermediate frequency signal IF1 by mixing the first pair ofdrive signals L01 and L02.

[0096] A second mixer 660 receiving the input signals RF1-RF4 generatesa second intermediate frequency signal IF2 by mixing with a frequency ofthe second pair of drive signals L03 and L04.

[0097] The first and second mixers 650 and 660 have the double balancedmixing circuit disclosed in the second embodiment of the frequencymixing circuit.

[0098] The present invention reduces the secondary intermodulationdistortion IMD2 by changing the input structure of the mixing circuitreceiving the radio frequency signal RF, so that removing the secondaryharmonic improves the linearity of the mixing circuit and quality ofreceiving circuit.

[0099] While the exemplary embodiments of the present invention havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe scope of the invention as defined by the appended claims.

[0100] For example, the third and other embodiments of the frequencymixing circuit can include the Gilbert cell circuit as the mixer. Themixer may include a Gilbert cell circuit, a folded-cascode circuit or aharmonic mixer circuit.

[0101] Additionally, the frequency mixing circuit and the radiofrequency receiving circuit may be fabricated via any known or futuredesign technology, for example, BJT, MOS, CMOS, BiCMOS, HBT, MESFET andHEMT, and may be formed on any known or future semiconductor substratesuch as Si substrate, SiGe substrate, GaAs substrate or InP substrate.

[0102] Furthermore, the transformer of the subtracter may be amonolithic microwave transformer on the semiconductor substrate that isknown as balun (balance to unbalance transformer).

[0103] The first voltage source may have a positive voltage level (e.g.,from 1V to 5V), and the second voltage source may have a negativevoltage level from negative value to ground.

[0104] The circuits of the present invention may be applied to acellular phone, a PCS (personal communication service) system, or a downconverter and up converter of radio frequency transceiver such as awireless LAN transceiver. The circuits of the present invention areadaptable to a direct conversion receiver of the cellular phone of a GSM(global system for mobile communications) having a frequency band of 900MHz, and to a direct conversion receiver of the PCS system of the GSMhaving a frequency band of 1,800 MHz and 1,900 MHz.

What is claimed is:
 1. A frequency mixer comprising: a firstdifferential amplifier for amplifying a first pair of input signals(RF1, RF2) having a first frequency (f1) to output a first differentialoutput signal; a second differential amplifier for amplifying a secondpair of input signals orthogonal to the first input signals to output asecond differential output signal, the second pair of input signalshaving a substantially same frequency as the first frequency (f1); asubtracter for subtracting the second differential output signal fromthe first differential output signal to output a subtracted outputsignal; and a mixer for mixing the subtracted output signal, and a firstpair of drive signals having a second frequency (f2) and a second pairof drive signals orthogonal to the first pair of drive signals andhaving the second frequency (f2), wherein the mixer has a sub-harmonicdouble balanced mixing mode, and for outputting a pair of output signalsorthogonal to each other, wherein at least one harmonic is cancelledfrom the output signals.
 2. The frequency mixer of claim 1, wherein thesecond frequency (f2) is about half of the first frequency (f1).
 3. Thefrequency mixer of claim 1, wherein the output signals belong to abaseband frequency.
 4. The frequency mixer of claim 1, wherein acancelled harmonic has a frequency of f1-2f2.
 5. The frequency mixer ofclaim 1, wherein the subtracter includes a first transformer and asecond transformer; the first transformer having a first polarity isinductively coupled to the first differential output signal of the firstpolarity and is connected between the first differential amplifier andthe mixer; and the second transformer is inductively coupled to thesecond differential output signal of a second polarity that is oppositeto the first polarity, and is connected between the second differentialamplifier and the mixer.
 6. The frequency mixer of claim 5, wherein thefirst differential amplifier has a first pair of emitter-coupledtransistors and a first bias current source; each of transistors in thefirst pair of emitter coupled transistors has a collector that isconductively connected to a respective one of the two terminals of afirst winding of the first transformer; a center tap of the firstwinding of the first transformer is connected to a first voltage source;the first pair of emitter-coupled transistors, wherein each transistorhas a bases respectively receiving one input signal of the first pairinput signals; and the first bias current source is connected between acommon node between the emitters of the first pair of emitter coupledtransistors and a second voltage source.
 7. The frequency mixer of claim6, wherein the mixer comprises; a first frequency multiplier connectedbetween a first output node and a terminal of a second winding of thefirst transformer, switched by the first pair of drive signals, a secondfrequency multiplier connected between a second output node and theterminal of the second winding of the first transformer, switched by thesecond pair of drive signals.
 8. The frequency mixer of claim 6, whereinthe first differential amplifier further comprises a pair ofregeneration resistors, each regeneration resistors being connectedbetween an emitter of each transistor and the common node between theemitters of the emitter-coupled transistors.
 9. The frequency mixer ofclaim 7 further comprising a capacitor connected between the firstoutput node and the second output node.
 10. A frequency mixercomprising: a first differential amplifier for amplifying a first pairof input signals having a first frequency (f1) for outputting a firstdifferential output signal; a second differential amplifier foramplifying a second pair of input signals orthogonal to the first pairinput signals having the first frequency (f1) and for outputting asecond differential output signal; a subtracter for subtracting thesecond differential output signal from the first differential outputsignal and for outputting a subtracted output signal; and a mixer formixing the subtracted output signal and a pair of drive signals having asecond frequency (f2) in a double balanced mixing mode for outputting apair of output signals orthogonal to each other wherein at least oneharmonic is cancelled.
 11. The frequency mixer of claim 10, wherein thefirst and second frequencies are substantially equal.
 12. The frequencymixer of claim 10, wherein the output signals have a baseband frequency.13. The frequency mixer of claim 10, wherein the harmonic has afrequency of f1-f2.
 14. The frequency mixer of claim 10, wherein thesubtracter has a first transformer and a second transformer; the firsttransformer is inductively coupled to the first differential outputsignal of a first polarity and is connected between the firstdifferential amplifier and the mixer; and the second transformer isinductively coupled to the second differential output signal of a secondpolarity that is opposite to the first polarity, and is connectedbetween the second differential amplifier and the mixer.
 15. Thefrequency mixer of claim 15, wherein the first differential amplifierhas a first pair of emitter coupled transistors and a first bias currentsource; the first pair of emitter coupled transistors has two collectorseach connected to a different one of two terminals of a first winding ofthe first transformer, respectively; a center tap of the first windingof the first transformer is connected to a first voltage source; andeach transistor in the first pair of emitter coupled transistors has abase respectively receiving one of the first pair input signals; and thefirst bias current source is connected between a common node of thefirst pair of emitter coupled transistors and to a second voltagesource.
 16. The frequency mixer of claim 15, wherein the mixer has athird pair of emitter-coupled transistors and a fourth pair ofemitter-coupled transistors, each transistor of the third pair ofemitter-coupled transistors has a collector respectively connected to afirst output node and a second output node, and a base receiving the oneof the pair of drive signals, respectively, and an emitter commonlyconnected to a terminal of a second winding of the first transformer;each transistor of the fourth pair of emitter-coupled transistors has acollector respectively connected to the first and the second outputnode, a base receiving one of the pair of drive signals, respectively,and an emitter commonly connected to a terminal of a second winding ofthe second transformer.
 17. A frequency mixer comprising: a differentialamplifier for amplifying a first pair of radio frequency (RF) inputsignals having a first frequency (f1) and for generating a first currentsignal at a first node and a second current signal at a second node; aharmonic rejection circuit for processing a second pair of input signalsorthogonal to the first pair of input signals and for generating a thirdcurrent signal at the first node and a fourth current signal at thesecond node, the second pair of input signals having a substantiallysame frequency as the first frequency (f1); and a mixer for mixing thecurrent signals applied to the first and the second node with a firstpair of drive signals and a second pair of drive signals beingorthogonal to each other and having a second frequency (f2), in asub-harmonic double balanced mixing mode, adapted to output a pair ofoutput signals orthogonal to each other, a harmonic being cancelled fromthe output signals.
 18. The frequency mixer of claim 17, wherein theharmonics rejection circuit comprises; a first transistor coupled to avoltage source via a first bias current source, wherein a base of thefirst transistor receives one signal of the second pair input signalsand an emitter of the first transistor is connected to the first node; asecond bias current source connected between the first node and aground; a second transistor coupled to the voltage source via a thirdbias current source, wherein a base of the second transistor receives aremaining signal of the second pair of input signals and an emitter ofthe second transistor is connected to the second node; and a fourth biascurrent source connected between the second node and the ground.
 19. Thefrequency mixer of claim 18, wherein the differential amplifier has afirst pair of emitter-coupled transistors and a fifth bias currentsource; each transistor of the first pair of emitter-coupled transistorshas a collector connected to the first and second nodes, respectively,and a base respectively receiving one of the first pair of inputsignals; and the fifth bias current source is connected between a commonnode of the first pair of emitter-coupled transistors and a secondvoltage source.
 20. The frequency mixer of claim 18, wherein a biascurrent of the fifth bias current source is about twice of a current ofthe first, second, third and fourth bias current sources.
 21. A circuitfor receiving a radio frequency signal comprising: a first poly-phasefilter for transforming a radio frequency signal having a firstfrequency (f1) into two pairs of input signals orthogonal to each other;a second poly-phase filter for transforming a local oscillator signalinto a first and second signals groups being 45° out of phase withrespect to each other, each of the signal groups having two pairs ofdrive signals orthogonal to each other; a first mixer for mixing the twopairs of input signals and the two pairs of drive signals in the firstsignal group and for outputting a first output signal having a thirdfrequency, the first mixer being operatively connected to the first andsecond poly-phase filters; and a second mixer for mixing the two pairsof input signals and the two pairs of drive signals in the second signalgroup and for outputting a second output signal having a substantiallysame frequency as the third frequency, the second mixer beingoperatively connected to the first and second poly-phase filters. 22.The circuit of claim 21, wherein the first and second output signals areorthogonal to each other.
 23. The circuit of claim 21, wherein each ofthe first and second mixers is a sub-harmonic double balanced mixeradapted to cancel harmonics.
 24. The circuit of claim 23, wherein thesecond frequency is about half of the first frequency.
 25. The circuitof claim 23, wherein the first and second output signals belong to abaseband frequency.
 26. The circuit of claim 24, wherein one of thecancelled harmonics has a frequency of f1-2f2.
 27. A circuit forreceiving a radio frequency signal comprising: a first poly-phase filterfor transforming a radio frequency signal having a first frequency (f1)into two pairs of input signals, each having a different phase fromother input signals; a second poly-phase filter for transforming a localoscillator signal having a second frequency (f2) into two pairs of drivesignals orthogonal to each other; a first mixer for mixing the two pairsof input signals with one pair of the two pairs of drive signals, tooutput a first output signal having a third frequency, the first mixerbeing operatively coupled to the first and second poly-phase filters;and a second mixer for mixing the two pairs of input signals with aremaining pair of the two pairs of drive signals, to output a secondoutput signal having a substantially same frequency as the thirdfrequency, the second mixer being operatively coupled to the first andsecond poly-phase filters.
 28. The circuit of claim 27, wherein thefirst and second output signals are 90° out of phase with respect toeach other.
 29. The circuit of claim 28, wherein each of the first andsecond mixers cancels harmonics.
 30. The circuit of claim 28, whereinthe first and second frequencies are substantially the same.
 31. Thecircuit of claim 28, wherein the first and second output signals belongto a baseband frequency.
 32. The circuit of claim 28, wherein acancelled harmonic has a frequency of f1-f2.
 33. A method comprising:transforming a radio frequency signal having a first frequency into twopairs of input signals orthogonal to each other; providing a first andsecond signal groups being 45° out of phase with respect to each other,each of the signal groups having two pairs of drive signals orthogonalto each other; mixing the two pairs of input signals and the two pairsof drive signals of the first signal group to output a first outputsignal having a third frequency; and mixing the two pairs of inputsignals and the two pairs of drive signals of the second signal group tooutput a second output signal having a substantially same frequency asthe third frequency.
 34. The method of claim 33, wherein the first andsecond output signals are 90° out of phase with respect to each other.35. The method of claim 33, wherein the two pairs of input signals aremixed with the two pairs of drive signals of the first signal group in afirst mixer, and the two pairs of input signals are mixed with the twopairs of drive signals of the second signal group in a second mixer, ina sub-harmonic double balanced active mixing mode adapted to cancelharmonics.
 36. A method comprising: transforming a radio frequencysignal having a first frequency into two pairs of input signalsorthogonal to each other; providing two pairs of drive signalsorthogonal to each other and both having a second frequency; mixing thetwo pairs of input signals and one pair of the two pairs of drivesignals to output a first output signal having a third frequency; andmixing the two pairs of input signals and a remaining pair of the twopairs of drive signals to output a second output signal having asubstantially same frequency as the third frequency.
 37. The method ofclaim 36, wherein the two pairs of input signals are mixed with one ofthe two pairs of drive signals by a double balanced active mixing modeadapted to cancel harmonics.
 38. The method of claim 37, wherein the twopairs of input signals are mixed with the remaining of the two pairs ofdrive signals by the double balanced active mixing mode adapted tocancel harmonics.
 39. A method comprising: providing a firstdifferential signal having a first frequency; providing a seconddifferential signal orthogonal to the first differential signal, andhaving a substantially same frequency as the first frequency;subtracting the second differential signal from the first differentialsignal to output a subtracted signal; and mixing the subtracted signal,and a first pair of drive signals and a second pair of drive signalsorthogonal to each other having a second frequency, in a sub-harmonicdouble balanced mixing mode, to output a pair of output signals beingorthogonal to each other, a harmonic being cancelled from the outputsignals.
 40. A method comprising: amplifying a first pair of inputsignals having a first frequency to output a first differential signal;amplifying a second pair of input signals orthogonal to the first pairof input signals to output a second differential signal, the second pairof input signals having a substantially same frequency as the firstfrequency; subtracting the second differential signal from the firstdifferential signal to output a subtracted signal; and mixing thesubtracted signal with a pair of drive signals having a secondfrequency, in a sub-harmonic double balanced mixing mode, to output apair of output signals orthogonal to each other, wherein a harmonic iscancelled in the output signals.
 41. A method comprising: amplifying afirst pair of input signals, having a first frequency, to output firstand second input signals; subtracting respectively a third and fourthcurrent signals from a first and second current signals to output afirst and second subtracted signals wherein a second pair of inputsignals orthogonal to the first pair of input signals have asubstantially same frequency as the first frequency; and mixing thefirst and the second subtracted signals, and a first pair of drivesignals and a second pair of drive signals orthogonal to each other, ina sub-harmonic double balanced mixing mode, to output a pair of outputsignals orthogonal to each other, wherein the first pair of drivesignals and the second pair of drive signals have a second frequency.42. A frequency mixer comprising: a first amplifier for amplifying afirst input signal having a first frequency, to output a first amplifiedsignal; a second amplifier for amplifying a second input signalorthogonal to the first input signal, to output a second amplifiedsignal, the second input signal having a substantially same frequency asthe first frequency; a subtracter for subtracting the second amplifiedsignal from the first amplified signal, and for outputting a subtractedsignal; and a mixer for mixing the subtracted signal with a drive signalhaving a second frequency, in sub-harmonic double balanced mixing mode,to output a output signal, a harmonic being cancelled from the outputsignal.
 43. The frequency mixer of claim 42, wherein the first andsecond amplifiers are differential amplifiers.
 44. The frequency mixerof claim 42, wherein the first and second input signals are 180° out ofphase with respect to each other.
 45. The frequency mixer of claim 44,wherein the drive signal has a first pair of drive signals and a secondpair of drive signals orthogonal to each other.
 46. The frequency mixerof claim 45, wherein the second frequency is about half of the firstfrequency.